(a) The Field of the Invention
The present invention has as its object a suction device for jet propeller assemblies for boats, which may be shortly called "hydrojets", as well as the propeller assemblies themselves.
(b) The Prior Art
As is known, and as will be better explained hereinafter, in the known jet propulsion boats (hydrojets) of the gliding keel or displacement type, a motor mounted in the boat controls through a transmission a pump assembly (which comprises an impeller which sucks the water as well as a guide casing and discharge nozzle through which the accelerated water is discharged, thereby producing the desired jet effect) which is mounted at a certain distance from the stern plate of the hull, and between the pump assembly and the hull a static device--which will be called "suction conduit"--is inserted, which has the function of permitting the suction of the water and its delivery to the pump assembly. The suction conduit begins with a suction inlet which, in the hydrojets presently manufactured, is inserted and penetrated into an opening in the hull bottom, and the aforesaid suction conduit is structured as a connection between the suction inlet and an inlet cross-section of the pump assembly, which guides the water stream from the former to the latter.
The hydrodynamic phenomena which occur in devices of this kind are extremely complex, and comprise viscous, turbulent and eddy phenomena. The thickness of the limit layer and the accelarations or decelerations which occur have a decisive effect on the efficiency of the propeller assembly. Without entering into theoretical details which would not be helpful in understanding the invention, it may be said that one of the preoccupations of the builders of hydrojets is to keep the flowing water stream from becoming detached from the walls of the suction conduit and to prevent possible cavitation phenomena. For this reason it has always been thought necessary to provide the aforesaid suction conduit with walls having an extremely mild and gradual curvature, as it was believed that otherwise the water stream would have become detached from the walls. As a consequence-as will be better explained hereinafter-the suction conduit has in the past been considerably extended in length. Further, the suction conduit penetrates into the hull, since, as has been said, the suction inlet is substantially an opening in the keel of the hull, and this invloves rather drastic construction and assembly limitations. Because of the rigid geometry of the resulting assembly, the position of the pump assembly with respect to the suction conduit is rigidly fixed as is the position of the conduit with respect to the hull.